a) Field of the Invention
The invention is directed to a liquid immersion microscope objective.
b) Description of the Related Art
Liquid immersion microscope objectives are used predominantly in the area of physiology where manipulation of living cells is performed in water. These cells and their reactions are then observed. Therefore, the liquid immersion microscope objectives used for this purpose must be designed in such a way that they can be immersed with their object-side lens in the cell culture liquid which is also the immersion liquid at the same time and, as a rule, mostly comprises water. Since living cells are worked with, the objectives must also permit integration of devices for preserving the life of the cells and must also leave sufficient free space for manipulation of the cell samples. Therefore, the objectives employed must be very slender. In the front, object-side area in particular, an angle of at least 35°—measured from the object plane—to the sample must be freely accessible to enable manipulation with different devices. A relatively large working distance of about 2 mm is likewise required.
Further, the samples to be examined are often so thick that sufficient examinations in visible light are impossible because the sample absorbs too much light. The desired resolution is not achieved until the infrared range.
Therefore, objectives of the type mentioned above must meet two contradictory requirements: on the one hand, the working distance must be as large as possible and the sample must be freely accessible at an angle of at least 35°; on the other hand, the numerical aperture must be as large as possible in order to achieve a high resolution.
An individual objective that meets these requirements, namely, the “CFI Fluor60×W” by Nikon, is known from the prior art. However, the chromatic correction in this objective is deficient. In particular, the longitudinal color aberration in the infrared range cannot be overcome to a sufficient degree and amounts to a multiple of the depth of field. The objectives disclosed in US 2003/0076600 and JP 8292374 have the same weakness.